There are a number of internal power consuming prosthetic devices now employed or contemplated for implantation in the human body. A common problem with all of these devices is providing an effective, safe power supply. Smaller devices such as pacemakers can use replaceable batteries. The necessity of surgically replacing batteries periodically is not a significant problem. With respect to devices requiring more power than the pacemaker battery supplied power is inadequate. Devices such as artificial hearts require up to 20 watts of continuous power. A battery designed to provide such power for 60 days would weigh at least 20 kilograms. Nuclear power supplies are also inappropriate since the shielding requirements would make these unsuitable. Further, if the shielding failed, the results could be catastrophic.
All devices used in humans for mechanical circulatory assistance have, therefore, required a permanent opening in the skin for energy transfer. These include pneumatic conduits for balloon pumps, the Jarvik total artificial heart and the Thermo Medical Systems assist device, blood conduits for the Thoratec (Pierce-Donachy) and Abiomed sacular pumps, an electrical cable for the Novacor solenoid pump and a spinning torque cable for the Nimbus Hemo pump intravascular turbine. Whereas these have occasionally been used for extended periods without infection (over two years in one Jarvik patient and over ten months in a current Novacor recipient), clinical and experimental observations indicate that such an integumental break presents a continuing risk of infection.
Principally for this reason, none of these devices except the Jarvik have been seriously proposed for long term circulatory support. All the others have been promoted as strictly temporary aid for use during expected cardiac recovery or during the waiting period for a transplant donor. Infections have been minimal for these short periods.
Electrical induction has long been entertained as a means of delivering power from extracorporeal source across intact integument. In 1961, a transformer operating with radio frequency alternating current from an external to a subcutaneous coil was reported by Schuder, Stephenson and Townsend. It was reasoned that a coil within a coil configuration could be more efficient and a tube pedicled skin flap was utilized. Within this tube of skin (shaped like a suitcase handle and attached to the chest wall at either end) lay a secondary coil while the primary coil with an iron core (allowing a lower frequency current to be used), surrounded it. Efficiencies of 97% (57 watts, 20 kHz) were reported.
Two groups of investigators have pursued these concepts for the past decade and have been developing inductive energy transmission systems seriously intended for powering of clinical circulatory assist devices. A belt skin transformer was developed by LaForge at Novacor which consists of a narrow single turn flexible secondary coil implanted in the subcutaneous tissue around the waist and a five turn extracorporeal primary coil worn in a belt. This has effectively transmitted 15 watts of continuous power at more than 75% efficiency in both in vitro models and experimental animal models. This system is intended to be coupled with a modified version of Novacor's current temporary solenoid operated intracorporeal left ventricular assist system to form a support system for long term use. Animals with the implanted device have survived for over two years with little difficulty reported.
An induction device has been developed by Thermedics, Inc. (now Thermo Medical Systems) which is situated in and on the anterior abdominal wall. The implanted secondary coil is made of 16 turns of braided copper wire wrapped around a dome-shaped polyurethane appliance within the abdominal subcutaneous tissue. The primary is a 3 turn coil in a ring that is worn surrounding the mound produced by the secondary appliance and secured by a belt. Transmission of 24 watts has been demonstrated. In efficiency studies in animals this has delivered 6 to 12 watts of usable power with a 3 watt loss (65%-70% efficiency). Most of the losses were demonstrated in external components and about 1 watt was lost in the transformer itself, presumably as heat. A clinical form of this device is to be used with an electric version of Thermo Medical System's current pneumatic left ventricular assist system.
These devices seem likely to offer a practical means for extra to intracorporeal energy transfer. Consideration of their use in patients, however, suggests some possible problems.
The only known prior use of a life supporting device that had to be maintained in the surface position was the radiofrequency induction coil used in pacemakers before introduction of satisfactory implantable batteries. These worked very well electrically (the very low power requirements of pacemakers needed a much less efficient inductor than assist devices) but there was a high fatality rate clinically due to inadvertent displacement by the patient.
Further, changes in electrical load of a pumping device or minor component failure in the activating circuit can potentially increase the heat produced in implanted secondary coils. Potential for dissipation of this heat without damage to surrounding tissue is limited by tissue blood supply. A serious burn of the tissue layer separating the primary and secondary coils could lead to device infection. Further, the discomfort and annoyance of a device that the patient can constantly feel in contact with his or her skin compounded by this psychological impact of knowing that that itch, tickle, or irritation is to be there for life is impossible to anticipate or calculate.